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November 4, 2003

Building Planets in Cyberspace

by Sam Savage

Jet Propulsion Laboratory -- Recipe: Take a rocky mass [about 12.8 thousand kilometers (nearly 8 thousand miles) wide], add carbon dioxide, water vapor and methane. Place in stable, circular orbit, the same distance from a sunlike star as the distance between Earth and the Sun. Heat to an average of 10 degree Celsius (50 degrees Fahrenheit) for 1 billion years.

Over the next few years, scientists at NASA's Jet Propulsion Laboratory plan to cook up a series of planets based on recipes like the one above and play around with the ingredients. But they won't be using real materials -- it will all be done in cyberspace.

The ultimate goal is to simulate a plausible range of habitable planets, and to find out how they might appear to planet-finding missions of the future.

Dr. Vikki Meadows is principal investigator of the Virtual Planetary Laboratory, a project that was selected as a new lead team for the NASA Astrobiology Institute to create tools that will simulate a diverse range of planets and life forms.

The Virtual Planetary Laboratory will marshal the best supercomputers available, and a team of 28 researchers from disciplines as varied as statistics and biology, to model a gallery of planetary atmospheres.

The team's findings will directly influence the development of future space missions such as Terrestrial Planet Finder, which will look for habitable planets around other stars.

"We're trying to build a terrestrial planet inside a computer," Meadows says. "This will help us determine what the signatures of life on an extrasolar planet will look like, once we have the technology to study them."

The closest planetary systems are many light years away, but the faint light the planets emit, if separated into its component frequencies, can provide a wealth of information. By analyzing the colors of radiation detected by Terrestrial Planet Finder, astronomers can look for the signatures of biological products.

These "biosignatures" can provide evidence that the environments on these planets may be able to support life. But what will these as-yet-unseen biosignatures look like? Finding out is part of the challenge, and that's where the Virtual Planetary Laboratory comes in.

Dr. Cherilynn Morrow of the Space Science Institute of Boulder, Colorado, a member of the team, says the Virtual Planetary Laboratory will help scientists know how to recognize habitable worlds and to discriminate between planets with and without life.

"The Virtual Planet Laboratory is playing a key role in defining how we will conduct our search for living worlds in orbit around other stars of the Milky Way galaxy," says Morrow, who heads the project's education and public outreach component.

Currently, scientists are limited to just one model of a habitable planet: Earth. The key to expanding our concept of what constitutes a habitable planet, Meadows says, is to play around with the recipe, trying different combinations of size, composition and location. A world teeming with microbes, for example, could produce an atmosphere rich in methane. And to learn about the plausible range of temperatures at which life might exist, "we'll model everything from frozen hells to burning hells," Meadows says.

To help scientists recognize younger Earths, the team will model our home planet as it would have appeared from space billions of years ago, before its atmosphere became rich in oxygen.

An equally important goal of the project is to learn how to recognize what Meadows calls "false positives" -- planets that may appear to have life, but don't. These planets would mimic some of the accepted signs of life, but would produce them using geological and atmospheric processes. Such planets might be distinguishable from inhabited worlds by looking at a broader spectral range, or taking many measurements over a period of time to understand the way these "signatures" change.

In the first phase of the project, the software will be used to re-create planets we're familiar with: Venus, Earth and Mars. Comparing the models with real data from observations of these planets will tell scientists whether the software is producing accurate simulations. Later stages will produce abiotic, or non-living, planets, and eventually, planets where life has found a foothold.

Meadows stresses that she and her colleagues aren't looking for "ET the Extraterrestrial." Their sights are set on life on a lower order -- even microbial. "I'm not looking for intelligent life," Meadows says. "I'm looking for bugs from space."